A variety of power conversion circuits have been developed for converting electric power at one voltage level or frequency to another voltage level or frequency. A commonly used device for providing an output voltage signal to a load at a different level than that of the available input voltage signal is a step-up or a step-down transformer, which may be provided either on the input or output side of a power conversion circuit. Such transformers are relatively large, heavy, and expensive. If a power conversion circuit is intended to maintain an output voltage at a desired level despite variations in the input voltage level (or to allow the output voltage to be adjusted to a desired level at a constant input voltage level) more complex transformer structures may be required. For example, some power conditioning circuits utilize tap-changing transformers, in which the primary transformer winding includes a number of separate taps which are connected to the power conditioning circuit input terminals by static switches. A controller circuit monitors the power conditioning circuit output voltage and switches the proper static switches on the input side to maintain approximately the desired voltage level on the output side. Tap changing transformers, however, require many switching devices, such as thyristors, and only permit coarse control of the output voltage. Furthermore, distortion of the voltage on a load cannot be well controlled using such a transformer.
Another type of line conditioning circuit structure utilizes a ferroresonant transformer. The ferroresonant transformer is connected in a circuit which provides passive compensation for short term changes in an input voltage level. Ferroresonant transformers are widely used, and provide reasonable voltage regulation, typically plus or minus 4% at the transformer output for a minus 20% to plus 10% change in the transformer input voltage. However, output distortion can be sensitive to load input harmonics, and the input currents drawn may have undesirable power factors and distortion levels when ferroresonant transformers are employed for voltage regulation.
Power conditioning circuits employing tap changing or ferroresonant transformers are limited in application by the presence of the low frequency transformer, which must be able to handle full rated power. For example, for a one kilovolt ampere (KVA) line conditioner employing such a transformer, the transformer by itself can weigh over 30 to 40 pounds, and is the major component determining the size of the cabinet containing the power conditioning apparatus.
Voltage boosting circuits for increasing an available AC input voltage level to a desired AC output voltage level without employing a tap changing or ferroresonant transformer have been developed. Such circuits typically require that the input AC voltage signal be fully converted to a DC voltage level on a DC bus. The DC voltage is then inverted to AC power utilizing a full bridge inverter circuit. Such systems typically require an inverter employing four switching devices to provide a single phase AC output voltage signal and an inverter employing six switching devices to provide a three phase AC output voltage signal.
U.S. Pat. No. 5,099,410, to Deepakraj M. Divan, describes a power conversion circuit which converts an AC source voltage at one voltage level to an AC output voltage at a different voltage level without using a transformer and with a minimum number of solid state switching devices employed. The basic power conversion circuit described in this patent includes a pair of rectifying devices connected together at a first node, a pair of capacitors connected together at a second node, and a pair of controllable switching devices connected together at a third node, with the pairs of rectifying devices, capacitors, and switching devices connected in parallel by DC bus lines. The three nodes serve as three terminals of the power conversion circuit. The first terminal or node is common to both the input power source and the load, the second node or terminal is connected to the source, and the third node or terminal is connected to the load. With the load connected between the first and third terminals and the source connected between the first and second terminals, the power conversion circuit can be controlled by a system controller to provide a peak-to-peak output voltage to the load which is substantially double the peak-to-peak input voltage. The rectifying devices charge the capacitors to the peak of the input voltage. To obtain effective voltage doubling, the system controller provides turn-on and turn-off signals to the controllable switching devices alternately and in phase with the input voltage so that the voltage across one of the charged capacitors plus the input voltage is applied to the load during one half cycle of the input voltage waveform and the voltage across the other charged capacitor and the input voltage of the opposite polarity is applied to the load during the other half of the input voltage waveform. With a low pass filter connected to filter high frequency switching components from the output voltage signal provided to the load, to thereby provide a substantially sinusoidal waveform, the circuit can be controlled to provide an output voltage signal across the load that can be varied from substantially zero to substantially twice the peak-to-peak input voltage. To obtain less than complete voltage doubling of the output voltage provided across the load, the switching devices are switched on for a duty cycle which is less than a full half cycle of the input voltage waveform. For example, pulse width modulation may be utilized in controlling the switching devices so that a desired effective AC voltage level is provided across the load. Only two switching devices are required to obtain such operation.
A common application for the various power conversion systems just described is to provide power line conditioning. Power line conditioning systems are used to provide compensation for and correction of voltage sags and drops in the signal provided on power lines to a load. Industrial process disruptions due to poor electrical power quality cause billions of dollars per year in lost productivity in the United States alone. Deep voltage sags and momentary voltage interruptions have been called the most important power quality concerns affecting most industrial and commercial electrical power customers. Productivity losses can occur, for example, when momentary electrical power interruptions cause factory robots to "lockup", resulting in entire production lines stopping and a significant amount of scrapped product. As another example, a momentary interruption of electrical power for as little as one cycle can cause high intensity discharge (HID) lighting to restart, resulting in a delay of as much as 20 minutes before factory lighting can be restored. The Computer Business Equipment Manufacturers Association (CBEMA) guidelines indicate that a short duration voltage sag to 75% of nominal for as little as 0.03 seconds can cause the malfunction of equipment.
Power line conditions which can result in productivity losses vary from long term power outages to short duration voltage sags. However, voltage transients and momentary power interruptions, due to events such as lighting strikes, and line under-voltages (voltage sags) down to no less than 45-50% of nominal voltage, due to faults on the utility power system, account for the vast majority, 90-95%, of all utility power quality related events in the United States. Momentary voltage interruptions typically last no more than three cycles, and voltage sags typically last 0.05-2 seconds. Such conditions are cleared rapidly as the utility finds a new route to source the power. Actual power outages that last more than two seconds are a very rare event in the United States and other developed countries.
Various solutions to the power quality problems encountered by utility customers are currently in use. For example, surge suppressors are employed to provide load voltage protection for very short duration voltage transients caused, e.g., by lighting strikes. Surge suppressors are very inexpensive, but provide no compensation for voltage sags or extended power outages. Relatively low cost voltage regulators, typically employing tap changing transformers, have been used to provide voltage sag compensation. Such voltage regulators are much more expensive than surge suppressors, and are only of limited effectiveness in responding to voltage transients, due to the slow response of such systems. The slow response of tap changing systems also limits their ability to provide correction of voltage sags. As discussed previously, a voltage sag lasting less than one cycle may cause an industrial system disruption. Moreover, low cost voltage regulators employing tap changing transformers are typically only available for the correction of voltage sags to no lower than 80% of nominal voltage levels. Such systems provide no protection for larger voltage sags or complete power outages. Higher cost voltage regulators, employing constant voltage ferroresonant transformers, have faster response times, and thus should be capable of providing correction for transient events. However, systems employing such constant voltage transformers typically only provide effective power line conditioning for voltage sags to no less than 70% of nominal voltage levels. Such systems do not provide any power correction for larger voltage sags or extended power outages. Additionally, power conditioning systems employing constant voltage transformers are much more expensive than systems employing tap changing transformers, and are also much more heavy and bulky.
Uninterruptible power supplies (UPS) have also been used to provide correction of power line voltage transients, sags, and drops. The typical UPS includes at least a battery and an inverter. When a power line voltage drop is detected, the power line is disconnected from the load, and the UPS inverter is turned on to provide AC power to the load from the battery until a normal voltage level is restored on the power line. The system battery is recharged by a rectifier, or by controlling the inverter to operate as a rectifier, with power from the power line when normal power line voltage is restored. A UPS can typically respond quickly to power line transients, voltage sags, and voltage drops, and can provide power to a load from the battery for several minutes, if necessary, in response to an extended power line outage. UPS systems are, however, relatively very expensive. Such systems typically employ many expensive power switching devices which are rated to operate to provide power to a load for an extended duration without interruption. The UPS battery is also typically very expensive, heavy, and bulky, and must be replaced periodically.
The prior art thus fails to provide a lightweight compact inexpensive power conditioning system with a rapid response time for providing correction of the vast majority of power line conditions which can cause productivity losses.